In a liquid crystal display device, a TFT substrate and a counter substrate are disposed opposite to each other with a liquid crystal interposed between the two substrates. The TFT substrate is a substrate in which pixels are arranged in a matrix form. Each of the pixels includes a pixel electrode, a thin film transistor (TFT), and the like. The counter substrate is a substrate in which color filters and the like are formed at locations corresponding to the pixel electrodes of the TFT substrate. With this configuration, the liquid crystal display device forms an image by controlling the transmittance of light of the liquid crystal molecules for each pixel.
There are various photolithography processes, in particular, in the formation of the TFT substrate of the liquid crystal display device. However, static electricity can easily be generated in the photolithography processes such as film formation and drying using a spinner. In the TFT substrate, a large number of scan lines and image signal lines intersect each other through an interlayer insulation film. Further, a large number of TFTs are formed to control these lines. When static electricity is generated in the manufacturing process, the interlayer insulation film is destroyed. As a result, a short circuit occurs between the scan line and the image signal line, or the TFT is destroyed. For this reason, the static electricity generated in the manufacturing process has a significant influence on the manufacturing yield.
In order to prevent the dielectric breakdown and TFT destruction caused by static electricity, measures are taken that prevent static electricity from entering a display area in which pixels and the like are formed, for example, by allowing the static electricity to flow to earth before entering the display area. JP-A No. 203761/2008 describes a configuration that protects a scan line drive circuit and the like provided in the vicinity of the display area, from external static electricity. In this configuration, a conductive film is coated on the surface of the scan line drive circuit and the like, and is grounded through an insulating film. Further, JP-A No. 21909/2001 describes an example of a diode circuit for protection against static electricity produced in the vicinity of the display area.
TFT using a-Si film is used in relatively large displays such as TV screens. On the other hand, TFT using poly-Si is used in relatively small displays such as mobile phones and portable game consoles. In the case of a liquid crystal display device using poly-Si TFT, the mobility of poly-Si is high, so that a drive circuit is formed by the TFT. Then, a scan line GL drive circuit and the like are mounted in a substrate.
The large liquid crystal display device using the a-Si TFT and the small liquid crystal display device using the poly-Si TFT are different in the configuration for protection against static electricity due to the difference in the space of the substrate. The large liquid crystal display device using the a-Si TFT has a relatively large space. Thus, a diode circuit is formed outside a display area 500 in which pixels are arranged in a matrix form, to protect the display area 500 from static electricity. FIG. 7 shows the circuit configuration.
In FIG. 7, a static electricity protection circuit is formed between the display area 500 and a terminal 200 coupled to a scan line GL. In FIG. 7, the static electricity protection circuit is formed using diode. The diode is formed by coupling a gate and a drain or source of a TFT. In FIG. 7, when large positive static electricity enters from the terminal 200, a diode 130 is turned on. When large negative static electricity enters, a diode 140 is turned on. This allows the static electricity to flow to earth, preventing destruction of the TFT in the display area 500, or preventing dielectric breakdown of an interlayer insulating film 300.
The small liquid crystal display device using the poly-Si TFT has a small substrate. In addition, a portion of the drive circuit formed by the TFT is mounted in the substrate. Thus, it is difficult to provide the static electricity protection circuit within the substrate. Such a small liquid crystal display device is manufactured by the following steps. First, a large number of substrates are formed on a mother panel. Then, the substrates are separated from the mother panel by scribing or other means. In this way, individual liquid crystal display devices are formed.
Thus, in the small liquid crystal display device using the poly-Si TFT, a small space is formed between the individual substrates, in which a static electricity protection circuit and the like are formed. At the time of scribing, the small space is removed and discarded. Because large static electricity is generated in the manufacturing process, the static electricity protection circuit is not used after completion of the product.
FIG. 8 shows the configuration of the static electricity protection circuit in the small liquid crystal display device described above. In FIG. 8, the substrate is located above a scribing line 210. Although the substrate includes the terminals 200 and the display area 500 or other components, only one terminal is shown in FIG. 8.
In FIG. 8, a static electricity protection line 230 extends beyond the scribing line 210 to the outside of the substrate. The static electricity protection line 230 is coupled to a diode 150 as well as a diode 160. Each of the diodes 150 and 160 is formed by coupling a gate and a drain or source of the TFT. Here, it is assumed that large static electricity is induced in the vicinity of the terminal 200. The diodes 150 and 160 are turned on to allow the static electricity to flow to earth. Thus, the TFT of the display area 500 or the interlayer insulating film and the like in the substrate is protected from the static electricity.
However, the higher the resolution demanded in the display the more the number of pixels. As a result, the number of scan lines GL, the number of signal lines DL and the like increase. For example, if the number of scan lines GL increases, it is difficult to form the static electricity protection circuit for each scan line GL in the limited space.
Meanwhile a selector driving method has been developed to address the increase in the number of scan lines GL due to the increased resolution. The selector driving method is a method for dividing the scan lines GL into a plurality of blocks, and scanning the scan lines GL for each block to reduce the number of leader lines of the scan lines GL. However, in the case of the selector driving method, the number of control TFTs increases to control the scan lines GL for each block. As a result, it is necessary to have a space for the control TFTs. Thus, the problem of lack of space for the static electricity protection circuit still remains.